1. Field of the Invention
The present invention relates to a communication system which performs non-contact proximity data transmission using near-field electromagnetic coupling effect produced between a transmission antenna and a receiving antenna disposed close to each other, and to an antenna apparatus used for such non-contact proximity data transmission. More particularly, the present invention relates to a communication system and an antenna apparatus which perform high-speed digital data transmission using near-field electromagnetic coupling effect.
2. Description of the Related Art
In recent years, in order to provide interfaces for processing a high-speed digital signal, there are standards, such as LVDS (Low Voltage Differential Signaling), XAUI (10 Giga bit Attachment Unit Interface), PCI (Peripheral Component Interconnect)-Express, etc. Some of the interfaces have a data rate of as high as over 6 Gbps. In these interface standards, a small voltage amplitude is employed in order to achieve high-speed signal transmission. However, there is a problem in that the interfaces are subject to more noise as the amplitude of voltage decreases. To overcome this problem, differential transmission is employed in place of single-ended transmission.
Among these, Low Voltage Differential Signaling (LVDS) has been developed for the purpose of reducing the number of signal lines, etc. For example, the number of signal lines necessary for transmitting a video signal having 6 bits to 10 bits for expressing individual gray scales of RGB is 20 to 40 by CMOS/TTL. Whereas by LVDS, the number can be reduced to 4 pairs (three pairs for data, and one pair for clock) to 6 pairs (five pairs for data, and one pair for clock). Main applications of LVDS include communication devices, PDPs (Plasma Display Panels), digital interfaces for liquid crystal panels, etc.
A differential transmission line controlled to have characteristic impedance of 100Ω is often used for a transmission line of a high-speed digital interface of this kind. A specific transmission line, which is employed in this case, includes a microstrip transmission line made of a dielectric substrate (printed-circuit board, etc.) having a conductor layer on a back side and a conductor pattern drawn by a line on a front side, a coaxial cable with a harness, etc. A transmitter IC (Integrated Circuit) and a receiver circuit are connected by a transmission line having a physical connection and an electrical connection as a matter of course.
As opposed to this, the present inventors think that it is possible to apply a method of high-speed digital signal transmission using a non-contact data communication technique. Non-contact communication has advantages that while data transmission is performed by radio, a transmitter and a receiver are disposed in proximity, and, thus, an intercepting device is not allowable to lie therebetween. Accordingly, secrecy may be maintained.
For example, two IC chips are mounted on one printed circuit board by flip chip attachment, and it becomes possible to perform data transmission using near-field electromagnetic coupling via transmission distances of 5.6 cm between the IC chips (for example, refer to Co-authored by Wilson J, Lei Luo, Jian Xu, Mick S., Erickson E., Hsuan-Jung Su, Chan B., How Lin, Franzon P., “AC coupled interconnect using buried bumps for laminated organic packages” (Electronic Components and Technology Conference, 2006. Proceedings. 56th, 30 May-2 Jun. 2006 Page(s):8 pp.); Co-authored by Lei Luo, John Wilson, Stephen Mick, Jian Xu, Liang Zhang, Evan Erickson, Paul Franzon, “A 36 Gb/s ACCI Multi-Channel Bus using a Fully Differential Pulse Receiver” (IEEE 2006 Custom Integrated Circuits Conference (CICC)). It is possible to achieve 2.5-Gbps data transfer by disposing an antenna electrode on the IC chip and an opposed antenna electrode on the printed circuit board, and connecting the IC chip with a transmission line on the printed circuit board using capacitive coupling between these electrodes. The sizes of antenna electrodes used here are 200 μm×200 μm for both the IC chip and the printed circuit board, and a communication distance is very short, namely 1 μm. Also, a bump is used for mounting the IC chip. That is to say, a bump formed on an IC chip is embedded on the printed circuit board, and thus both of the antenna electrodes are disposed in close proximity, which is very complicated. The IC chip is mounted by flip chip attachment, and, thus, it is difficult to detach or to replace the IC chip after the mounting.
Also, as another example of a non-contact data transmission technique, a proposal has been made of a technique of transferring data between chips produced by a laminated plurality of IC chips, which are polished as thin as tens of micrometers in consideration of SIP (System In Package) implementation (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-228981; Co-authored by Miura N., Mizoguchi D., Inoue M., Sakurai T., Kuroda T., “A 195-gb/s1.2-W inductive inter-chip wireless superconnect with transmit power control scheme for 3-D-stacked system in a package” (Solid-State Circuits, IEEE Journal of Volume 41, Issue 1, January 2006 Page(s):23-34); and Co-authored by Jian Xu, John Wilson, Stephen Mick, Lei Luo, Paul Franzon, “2.8 Gb/s Inductively Coupled Interconnect for 3-D ICs” (2005 Symposium on VLSI Circuits Digest of Technical Papers)). For example, a plurality of channels including a transmission and receiving circuit, and an antenna coil are laid out on an IC chip at 50-μm intervals in proximity using a semiconductor process. When an antenna coil having a diameter of 48 μm is used, it is possible to achieve 1.0-Gbps data transfer between antennas that are 43 μm apart.
Here, non-contact data transmission techniques using near-field electromagnetic coupling can be roughly divided into techniques of using capacitive coupling between two antenna electrodes provided at a transmitter and a receiver, respectively, and techniques of using inductive coupling between two antenna coils in the same manner. Also, the above techniques can be divided into two kinds of techniques from another viewpoint. One of the techniques does not necessitate impedance matching in accordance with a length of a wire connecting a transmission and receiving circuit, and an antenna. The other techniques necessitate impedance matching.
When an antenna is disposed very near to a transmission circuit or a receiving circuit, an input/output terminal of the circuit and an input/output terminal of the antenna operate in a substantially same phase, and thus the influence of reflection can be disregarded. Accordingly, impedance matching is not always necessary. In contrast, if an antenna is disposed apart from a transmission and receiving circuit, a length of a wiring line between them (transmission line) can not be disregarded, and thus impedance matching becomes necessary between an input/output terminal of the circuit and an input/output terminal of the antenna. In particular, in the case of high-speed data transfer exceeding 1 Gbps, if there is an impedance mismatch in a system including a transmission and receiving circuit and an antenna, reflection is caused by the mismatch. Accordingly, unnecessary ringing occurs on a receive signal, which causes an increase in jitter and deteriorates an error rate. Thus, high-speed data transfer is hindered.
In the case of capacitive coupling, if an antenna electrode has a length not less than ⅛ times a signal wavelength λ (in consideration of a wavelength contraction ratio), it is necessary to consider a resonance frequency depending on the length. Also, if a parasitic inductive component (L) of a feed line is not disregarded, the parasitic inductive component and a self-capacity (C) of an antenna electrode form a series resonant circuit, and there is a self-resonant frequency fr to be determined by ½π√LC. In contrast, only in the case where the antenna size is sufficiently smaller than λ/8, and the above-described parasitic inductive component can be disregarded, the circuit can be regarded to have a pure capacity. Accordingly, the coupling of the transmission and receiving antennas can be regarded as a so-called AC coupling.
On the other hand, in the case of inductive coupling, an inductive component (L) of a coil and a parasitic capacitive component (C) of a wiring line forming the coil and with respect to GND form a parallel resonant circuit, and there is also a self-resonant frequency fr to be determined by ½π√LC in this case.
In a frequency band not less than the self-resonant frequency fr, the capacitive coupling antenna does not function as a capacitor, and the inductive coupling antenna does not function as an inductor. Also, resonance occurs at a signal component near fr both in the capacitive coupling antenna and in the inductive coupling antenna, and thus a frequency band that can be used for data transfer is restricted by the self-resonant frequency fr.
To date, for a non-contact data transfer antenna, a so-called lumped-parameter antenna structure has often been employed. In general, a large-sized antenna tends to have a low self-resonant frequency fr. Thus, in order to allow the use of a high frequency band and to increase a data transfer rate, it is necessary to set the size of the antenna small. However, in the case of non-contact communication using near-field electromagnetic coupling, a communication distance thereof becomes the same level as the antenna size. Accordingly, if a small-sized antenna is used, there is a restriction that a transfer distance also becomes short.
In this manner, in a related-art non-contact communication, there is a drawback in that the transfer distance becomes short when data is transferred at a high speed. Thus, applications of non-contact communication is limited to an ultra short distance, such as data transfer between laminated IC chips, etc. Also, if an antenna is disposed apart from a transmission/receiving circuit, and is connected to the circuit by a transmission line, a data transfer rate is limited to about ½ times an antenna band in the case of a resonant narrow-band antenna. Accordingly, there is a drawback in that it is difficult to achieve high speed.